Selective circuits



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UNITED STATES PATENT OFFICE.

MAURICE B. LONG` O1" EAST ORANGE, NEW' JERSEY, .AS'SIG'NOE TO' WESTERN ELECTRIC' COMPNY, INCORPORATED, OF NEW' YGR-K, N. Y.. it CORPORATON Gli" NF'W SELECTIVE CIRCUTLS.

Application filed .Tune l', 1922. Serial No. 563.945.

To all @0.7mm t muy 'con c' ern Be it linown that l, ironici: l. Lone, a citizen of the United States of America. residing atlilast Orange. inthe county of lsse: and Stu-te of New Jersey, have invented certain new andfuseful Improvements in Se lective Circuits, of which the following is a full,- clear, concise, and exact description.

This invention relates to improvenients for` selectively receiving and transmitting` alternating currents.

In a copending application of Paul H. Pierce, SerialI No. 484,342, filed July 13, 1921, assignedr to the assignee of this application, there is described' a methodof and means for selectively receiving alternating currents of which this invention is an improvement.

An object of the present invention is to maintain high selectivity in a tuned circuit associated with a translating device.' ln a multiplex signaling system it is desirable to maintain a high degree of selectivity in the tuned circuits which separate the several transmissions and which are located between the transmission line and the terminal translating devices, in order to provide as many transmission channels as possible within a given frequency range. A. second object of this invention is to provide highly selective circuits for suoli aV system. Other objects will become apparent 4from the description to follow.

According to this' invention a loop timed circuit is associated with 'a translating device by means of an energy modifying or transforming means. ln one embodiment of the invention this' means` comprises ar stepdown transformer having its high impedance winding connected to the timed loop and its low impedance winding, to the translating device. ln a more specific embodinjient of thelinvention the receiving circuit for a n'iultiplev` 'carrier current telegraph system comprises a loop tuned circuit supplying energy to the input circuitof a therinionic vacuum tube amplifier through an inequalityimpedance ratio step-down transformer. The input impedance of the amplifier, when viewed from the high Yimpedance winding of the transformer, appears larger than' is actually the case. Since it isY this winding', which is connected in shunt to a. portion of the tuned loop, less energy loss is caused by this arrangement than if the amplifier itself were directly connected tothe tuned loop.

This invention also contei'npl'ates the use of other energychanging or transforming devices 'equivalent toL the step-down transformer vit also contemplates the substitu-` tion of' other translating devices, such as electron#discharge detectors, rectifiers of various liinds and like devices.

The invention will now be' further described withV reference to the drawing in which Fig. l shows one embodiment thereof in a. carrier wave signaling system; Figs; and 3 show graphicallythe results of tests which are `evidence of the improvement pro-` vided by this invention; and'v Fig. 4t :is a modified circuit arrangement showing the invention embodied in 'a transmitting circuit.

Referrii'ig now to Fig. l, the carrier wave signaling system, therein' shown, comprises a transmission line ML to which a plurality of transmitting stations are connected through the balance-d' transformer 5 vwith its associated balancing network NW and to which a plurality of cooperating receiving stations areconnected through the'bal'anced transformer 6 with its associated network NE. Each transmitting station comprises an oscillatorO,y an amplifier TA and a loop tuned circuit LW comprisin'g'a lumped' inductance 7, the1 'primary winding of transl former 9 anda condenser 8. This' loop timedA circuit LV is loosely coupled through transformer 9 to aV series tuned circuitconn prising a capacity element l() and the inductive coils il. licon'stituting the. secoi'idary winding of the transformer 9. This series tuned circuit in turn is connected through thev common transmitting circuit TCV to the balanced transformer 5. The oscillator O is normally activebut forsignaling purposes oscillations are prevented from being transmitted to the common line ML by a short circuit for the oscillations at the ampliiier TA. This short circuit is controlled by the key K in series with a largel capacity which is of negligible impedance foi-'the oscillations produced by source 0.

rl'he oscillator O may be of any suitable type, preferably an electron discharge oscillator such as is described in Trlartley Patent No'1,356,763, October 26, 1920. Likewise, the amplifier TA comprises preferably an electron discharge device in general like the high frequency amplifiers HFA1 and HFA?, hereinafter to be described more in detail, but it may be of any other suitable tyDe Each of the plurality of transmitting channels is similarly arranged and connected with the common line ML through the transformer 5 and common transmitting circuit 'PCi/V.

Each receiving circuit comprises a series tuned circuit having capacity element 12 and inductive elements 13, 13, connected to the common line ML through the common receiving circuit RCE and the balanced transformer (3. Loosely coupled to this series circuit, through the transformer 141, is a loop tuned circuit comprising the secondary winding of transformer 111, an inductive element 15 and a capacity elements 16. Connected to this loop tuned circuit, through a step-down transforn'ier 17, is a high frequency amplifier HFA1 connected through a potentiometer 1S to a second high frequency amplifier HFA@ which in turn is directly connected to a detector D. Receiving device 19, responsive to .unidirectional current from the detector D is connected thereto.

rlhe high frequency amplifiers HFAl and HFA2 and the detector D are preferably of the well known three element electric discharge type. The an'ipliiier HFAl has a lilament 20, a grid 21 and a plate The amplifier HFA2 has a filament 23, a grid and a plate 25. The detector D has a filament 26, a grid 2T and a plate 2S. The three lilaments 20, 23 and 20 are connected serially with a battery 29 and a resistance 30. The function of this resistance is to normally maintain the grid 21 at a small negative potential with respect to the filament 20. Between the filament and the plate of each device is a battery 31. Between the battery 81 and the plate 22 of high frequency amplifier HFAl and the plate of the high frequency amplifier HFA2 are connected respectively choke coils 82 and he function of these coils is to permit the flow of direct current while substantially' preventing the flow of any alternating current. Receiving device 19 is connected in series with the battery 31 between the plate 28 of the detector D and the series filament circuit. A condenser t is connected in shunt of the receiver 19 to improve the operation of the detector D in a manner which is well known. The stopping ccndensers 35 and 36 are provided to prevent the 'liow of direct current from the battery 31 to the potentiometer 1S and the resistance 38 associated respectively with the high frequency amplifier HFAZ and the detector D. Between the series filament circuit and the grid 2T of detector D is connected a battery 3T and the high resistance 38. The battery 37 has its negative terminal connected to the grid 2T through the high resistance 3S, which with condenser 36 acts as a grid leak, the operation of which in connection with a detector is well known and therefore needs no further description.

The high impedance winding t0 of transformer 17 is connected to the terminals of condenser 16 in the loop tuned circuit LE. T he low inipedance winding 41 of the transformer 1T is connected to the input of the high frequency amplifier HF A1. Due to this arrangement the input impedance of the high frequency amplifier HFrtl, appears much larger than it actually is, when viewed from the terminals 42, 43 of the high impedance winding, which terminals are connected to the loop tuned circuit LE. The sharp ness of timing of the loop LE is therefore little affected by the other apparatus in the receiving circuit.

Each of the plurality of receiving chalinels is similarly arranged and connected to the common line ML through the transformer (3 and the common receiving circuit RCE.

1t is understood that a distinctive carrier frequency is employed for each transmitting channel and its cooperating receiving channel and that the constants of the tuned circuits of the several channels will necessarily be different in order that each of the dif-A ferent carrier frequencies may be conlined to its proper channel. 1t is also to be understood that a plurality of receiving channels may be connected to the balanced transformer 5 by means of a common receiving circuit RCW and that a plurality of transmitting circuits may be connected to the balanced transformer 6 by means of the common transmitting circuit TCE. For simplicity, such channels are not shown in the arrangement of Fig. 1.

Referring now to Figs. 2 and 3, Fig. 3 shows the selectivity of the receiving circuit of Fig. 1 for various frequency spacings and signal frequencies. Fig. 2 shows the same data with respect to a circuit such as disclosed in the above mentioned copending Pierce application.

The abscissae in each ligure represent the frequency spacing between channels in cycles per second. numbers which are a measure of the selectivity of the individual circuit. The numbers opposite the several curves show the frequency to which the receiving circuit was tuned when the dat-a for that curve was obtained. For example, assume that the receiving circuit- LE shown in Fig. 1 is tuned for 7000 cycles per second. Other similar receiving channels are tuned for frequencies on either side at intervals of /100 cycles. At the transmitting station are sources arranged to supply currents of these frequencies to the common line ML. Under these eircun'istances the selectivity shown by the intersection of the curve labeled 7000 The ordinates in each case are' with the ordinate marked 400 kis slightly over 20, which means that 'the effect of the detector D associated with circuit LE of current of `7000 cycles is approximately twenty times as great as the maximum effect of any one of the other frequencies. For the same frequency spacing of L100 cycles and signal frequency of 7000 cycles, the selectivity shown by Fig. 2 at the intersection of the cur-ve labeled 7000 with the ordinate marked 1100 is about 16, or about twenty per cent less. Cr, in order to obtain a selectivity of 20,a frequency spacing of about 450 cycles would be required in the Pierce system.

From these results it is seen that the receiving circuit of this invention provides a decided improvement as to selectivity for a given frequency spacing or frequency spacing for a. given selectivity over previously known receiving circuits. This improvement in selectivity or reduction in frequency spacing is obtained moreover with only a negligible sacrifice in efficiency. It may also be noted in this connection that in actual commercial practice the use of the circuit of this invention has made it possible to obtain ten duplex telegraph transmission channels within the frequency range required previously for eight such channels.

In the commercial system just referred to the transformer 14 has 660 turns in the high impedance winding 13, 13, and 17 turns in the low impedance windings or an impedance ratio of approximately 1500 to 1. Inductive element 15 consists of a double winding toroidal coil having a total of 660 turns. The inductance of this coil at 1000 cycles per second is 150 millihenrys and at 7000 cycles per second 160 millihenrys. The

c: pacity of the condenser 16 is adjusted to approximately .00323 microfarads for the 7000 cycle channel in order to make the circuit resonant at that frequency. High impedance winding 10 of transformer 17 consists of 5500 turns and the low impedance winding 41 of 1900 turns giving an impedance ratio of 8.5 to 1.

ln the modification of Fig. t the invention is applied to a transmitting circuit which may be substituted for the transmitting circuit of Fig. 1 if desired. Like reference characters are used to designate like elements in both Figs. 1 and f1.

The oscillator O supplies energy to the loop tuned circuit LV through the electron discharge amplifier TA1 and the inequality impedance ratio transformer 50. rllhe low impedance winding 53 is connected to the output circuit of the amplifier TA1 and the high impedance winding 54;, to the loop tuned circuit LV at the points 51, 52. The relatively low impedance of the out-put circuit of the amplifier TA1 therefore appears as a relatively higher impedance when viewed from the points 5.1,y 5210ithe 4loop tuned` circuit LW. As previously. explained in connection with the receiving circuit of Fig.` 1 such a connection maintains the selectivity of the loopY tuned circuit.

Although this invention has been specifically described as embodied ina carrier wave signaling system, it is tolbe understood that it is applicable to selective circuits lgen-'- erallfy without regard. tothe purpose for whichthey areused. i Theinvention` is there-1 fore to be-limitedv only by the scopeof the appended claims. Y

That is claimed is:

1. A current wave transmission system comprising a loop tuned circuit, a translating device, and a step-down transformer connected between said loop tuned circuit and said translating device for the transfer of energy therebetween.

2. A current wave receiving system comprising a loop tuned circuit, a detecting device receiving energy from said loop, and a step-down transformer connected between said loop tuned circuit and said detecting device.

3. A selective electric wave transmission system comprising a loop tuned circuit, a translating device, and an impedance changing device so connected between said tuned circuit and said translating device that the impedance of the latter as measured from the point of connection to the former is increased.

4. 1n an electric wave receiving system, an inductive element and a capacitative elementv connected in parallel with each other to form a tuned circuit, a detecting device, and an inequality impedance ratio transformer having the terminals of its high impedance winding connected to electrically separated points of said tuned circuit and the terminals of its. low impedance Winding connected to said detecting device.

A multiplex signaling system comprising a plurality of different frequency wave transmitting circuits, a plurality of cooperating` receiving circuits, a common transmission medium uniting said transmitting and receiving circuits by means of whic the different frequency waves are conveyed from the transmitting circuits to the receiving circuits, and a loop tuned circuit a step-down transformer and an electron ischarge device connected in series with each. of said receiving circuits.

ln a multiplex signaling system, a common transmission circuit, a plurality of receiving` circuits connected thereto each primarily responsive to a frequency different from the others, each said receiving circuit comprising a loop tuned circuit, a stepdown transformer having its high potential terminals connected to said loop tuned circuit, and a translating device having its input circuit connected to the low potential terminals of said step-down transformer.

7. In a multiplex signaling system a common transmission circuit, a plurality of receiving Circuits connected thereto each primarily responsive to a frequency different from the others, each said receiving` circuit Comprisinp` a loop tuned Circuit, a stepdOWn transformer having its high potential terminals connected to said loop tuned circuit, and an electron discharge amplifier havingits input oireuit connected to the lou7 potential terminals of said step-down transformer.

8. A selective alternating current receiving circuit comprising an inductive coil and a condenser connected in parallel, a stepdown transformer having its high potential terminals Connected to the terminals of said condenser, and an electron discharge device having an input and an output eircuit7 said input circuit being connected to the low potential terminals oifrsaid stepdown transformer and the output circuit associated with a signal receiving device.

Vln witness whereof, l hereunto subscribe my naine this 16th day of Jiuie A. D. 1922.

MAURICE is. Lone. 

